Soil fertility
experiments will help ensure sustainable use of sewage sludge on land
DEPARTMENT FOR
ENVIRONMENT, FOOD AND RURAL AFFAIRS News Release (News Release ref :
419/07) issued by The Government News Network on 14 November 2007
An investigation
into the effects on soil quality and fertility of the heavy metals
present in sewage sludge has published interim findings today. The
results will help inform the sustainable recycling of organic
material to land to improve soil quality.
Spreading treated sewage sludge on agricultural land provides
organic matter and nutrients and maintains soil carbon, as well as
offering what is in many circumstances the best environmental
option for recycling of sewage sludge. The alternative disposal
routes are landfill or incineration, both of which waste a
potentially valuable resource.
Experiments started in the mid 1990s to examine the effects of
cadmium, zinc and copper in sewage sludge on soil micro-organisms.
The aim was to investigate long term impacts, and the metal-rich
sludge was applied at elevated rates over 4 years to establish
treatments that exceeded the maximum permissible soil metal
concentrations detailed in the Sludge Use in Agriculture
Regulations. In operational practice, it would take longer than a
100 year period to reach these concentrations.
Headline results show that under certain conditions metal-rich
sludge could impact upon some microbial groups, with implications
for soil quality over the long term. The significance of the
results will be considered in light of several factors, including:
* The intensity of the sludge application in the experiments
would not happen in the field, and the results need to be
interpreted with caution and in light of modern practice;
* The amount of metals entering the sewerage system has declined
sharply in recent years, and the sludges used in the experiments
do not represent current quality. The results therefore represent
the potential impacts under a worst-case scenario; and
* The long timescale over which the effects were projected means
that there is good time, before there is any significant risk of
damage occurring, to properly investigate the issues arising from
the experiments so far. It is important that this work is taken
forward carefully to ensure that policy on sewage sludge spreading
is based on sound science.
Further work is now planned to understand the mechanisms behind
the identified impacts and how these could be influenced.
The interim results are published at:
http://randd.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&ProjectID=10677&FromSearch=Y&Publisher=1&SearchText=SP0130&SortString=ProjectCode&SortOrder=Asc&Paging=10#Description
Notes to editors
1. A Review by an Independent Scientific Committee on the Rules
for Sewage Sludge Application to Agricultural Land - Soil
Fertility aspects of potentially toxic elements (MAFF, 1993)
recommended that "further research was needed to examine the
effects of heavy metals from sewage sludge on soil micro-organisms."
2. The experiment was set up in 1994 to determine the effects of
sewage sludge additions to land on soil microbial populations and
on soil function. Soil micro-organisms are the portion of soil
responsible for the breakdown of organic materials and nutrient
cycling. The trials have no relevance for health and safety
aspects of sewage sludge recycling.
3. The Defra research project was co-funded by the Scottish
Executive, Welsh Assembly Government, Environment Agency and UK
Water Industry Research.
4. The regulations are: The Sludge (Use in Agriculture)
Regulations 1989 (and the 1990 amendments thereto). These are
supported by a departmental Code of Practice for Agricultural Use
of Sewage Sludge (latest version 1996), which recommends a tighter
soil limit for zinc.
5. Since the trials began, changes in industrial practices and
strict controls at source have led to considerable reductions in
the concentrations of metals found in sludge from sewage works.
The main inputs of metals now come from diffuse sources such as
run-off and household use rather than industrial sources.
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